1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly to semiconductor devices manufactured by using conductive resist films as masks in ion implantation. The invention further relates to a method of manufacturing such semiconductor devices.
2. Description of the Background Art
In the manufacture of a semiconductor integrated circuit, when a transistor or the like is formed on the surface of a semiconductor substrate, P type or N type impurities need be diffused in the semiconductor substrate. Ion implantation is employed as one method of diffusing the impurities into the semiconductor substrate. As described in an article entitled "ION IMPLANTATION" by T. E. SEIDEL in VLSI TECHNOLOGY, chapter SIX, pp. 219-265, edited by SZE, the ion implantation method is a method of ionizing such as gas including impurities, selectively taking out the required ions of the ionized gas by mass spectrometry employing an electric field/magnetic field, accelerating the ions by the electric field to irradiate the semiconductor substrate, thereby implanting the impurities into the semiconductor substrate. This method is an indispensable technology for an LSI (Large Scale Integrated Circuit) with increased high performance due to high accuracy in the control of the amount of impurities and of the thickness of impurity layers to be formed. As one of the ion implantation methods for the LSI, a method in which photoresist is used as a mask in implantation is known.
A method of manufacturing a conventional semiconductor device, to which such method as described above is applied will be described with reference to FIGS. 8A-8E.
Referring to FIGS. 8A and 8B, an insulating film 2 is formed on a silicon substrate or a semiconductor substrate 1. Next, referring to FIG. 8C, a resist film 3 is formed on the insulating film 2, and by employing photolithography and etching, the insulating film 2 and the resist film 3 which are positioned in a region to be an active region are removed to form an opening portion 4. Referring to FIG. 8D, ions such as boron or phosphorus are implanted in the semiconductor substrate 1 through the opening portion 4. Accordingly, an active region 6 is formed in the semiconductor substrate 1. At this time, ions 5 also enter the resist film 3. Since the resist film 3 is electrically insulated by the insulating film 2 from the semiconductor substrate 1, a charge 7 becomes stored in the resist film 3 as ion implantation proceeds, as shown in FIG. 8E.
The above described process of ion implantation is carried out in a high vacuum chamber. Since this vacuum chamber is grounded, the semiconductor substrate is at the ground level. Further, there exist in the chamber ion species incident on and directed onto the semiconductor substrate to be a target. Consequently, the charge stored into the resist film 3 is transferred to the semiconductor substrate or chamber with the ion species, and is then spontaneously discharged out of the chamber. Since a decrease in the charge due to the above described spontaneous discharge is, however, extremely slow, a large portion of ions implanted in the resist film 3 remains therein. Therefore, the ions or charges increase as the time of implantation increases.
To increase productivity recently, as the current value of ions increases from a conventional value of 500 .mu.A/cm.sup.2 through 1 mA/cm.sup.2 to several mA/cm.sup.2 through 10 mA/cm.sup.2, the charge stored in the resist film 3 increases considerably. Further, since the semiconductor substrate 1 is normally at the ground level, a potential difference between the resist film 3 and the semiconductor substrate 1 sharply increases. As described above, as the potential rises, discharge occurs in the end portion 300 of the resist film, with the end portion of the film 2 having the lowest breakdown voltage, sandwiched between the resist film and the substrate. At this time, the charge stored in the resist film 3 rapidly flows into the semiconductor substrate 1 through this discharge portion, so that the semiconductor substrate positioned beneath the end portion 300 of the resist film, particularly the active region 6 is destroyed. In fact, when the resist film 3 has a potential of several hundred volt for the semiconductor substrate 1 by charging, a high electric field occurs between the resist film 3 and the semiconductor substrate 1, so that a hole is made in the insulating film 2 sandwiched therebetween due to discharging. This phenomenon is called dielectric breakdown. As a degree of integration of the semiconductor device becomes increased, the insulating film 2 becomes progressively thinner, so that the dielectric breakdown is liable to occur.
To avoid the dielectric breakdown, time-consuming ion implantation has been carried out with an ion implantation current corresponding to a very little discharge current, resulting in a decrease in productivity.
Referring to FIG. 9, Japanese Patent Laying-Open No. 63-58824 discloses a process for ion implantation comprising the steps of forming a photoresist film 3a in a region of a semiconductor substrate 1 to be masked, forming a conductive thin film 3b made of charge transfer-type organic conducting materials in the entire surface of the semiconductor substrate 1, and implanting ions on the conductive thin film 3b into the semiconductor substrate 1 so as to form an ion implantation region 6a.
Japanese Patent Laying-Open No. 58-96732 discloses another process for ion implantation comprising the steps of forming a photoresist film on a prescribed region of the semiconductor substrate, forming an Al film on the photoresist film, the portion of which is electrically connected to the semiconductor substrate, and implanting ions with the Al film and the photoresist film used as masks.
Japanese Patent Laying-Open No. 60-116128 discloses that a conductive film is formed on the entire surface of the semiconductor substrate before ion implantation.
According to these disclosures, applying the charge stored in the photoresist film to the grounded semiconductor substrate through the conductive film prevents the photoresist film used as a mask from being charged. However, a processing step of forming the conductive film is required, so that there is a problem that the number of the processing steps increases.
As has been described, there exists a problem in a conventional method of manufacturing the semiconductor device that in order to prevent the resist film used as a mask from being charged, the time-consuming ion implantation and the processing step of forming the conductive film are required.